a. Field of the Invention
This invention relates to a method of and a device for enabling an image bearing member such as transfer medium or the like to be conveyed without disturbing an unfixed visible image electrostatically retained thereon in an image formation apparatus such as electrophotographic apparatus or electrostatic recording apparatus.
b. Description of the Prior Art
Generally, in an image formation apparatus such as electrophotographic apparatus or electrostatic recording apparatus, image formation is accomplished by developing an electrostatic image into a visible image. Where electrophotography is used, a photosensitive medium having a photoconductive layer is employed as an electrostatic image bearing member. Such photosensitive medium, i.e. electrostatic image bearing member, is subjected to charging by a corona discharger or the like and if required, voltage application for removal of the charge, exposure to light information, etc., thereby forming an electrostatic latent image. Such electrostatic latent image is developed into a visible image while being retained on that electrostatic image bearing member or after having been transferred to another electrostatic image bearing member. The image development is effected either by the dry type developing technique using dry particulate developer or by the wet type developing technique using developer particles diffused in a carrier liquid. The visible image so formed on the electrostatic image bearing member is transferred to a transfer medium such as plain paper superposed on the image bearing member, by applying a voltage to the back side of the transfer medium from a voltage applying means such as corona discharger or roller electrode. The transfer medium, after having the visible image transferred thereto, is separated from the electrostatic image bearing member and conveyed to fixing means including heaters and heating rollers for fixing the visible image, whereby the visible image is fixed. Since the visible image on the transfer medium so separated and conveyed to the fixing means is in its unfixed state, it is necessary to protect the unfixed visible image against disturbance during the conveyance of the transfer medium. However, with an image formation apparatus, for example, an electrophotographic copying apparatus which tends to be of a reduced size and is a complicated mechanism, it is often the case that the conveyance path of the transfer medium is unavoidably formed with a bend. If the unfixed transfer medium is conveyed along a conveyance path having such a bend, the visible image thereon may sometimes be disturbed, thus failing to provide a good copy image. The reason why such phenomenon of disturbance occurs to the visible image may be appreciated from the following:
First, in the step of transferring a visible image from an electrostatic image bearing member to a transfer medium, it is usually the practice that charge of the opposite polarity to the developer is uniformly applied to the back side of the transfer medium from a corona discharger or the like. By this, charge of the opposite polarity to the charge from the image transfer corona discharger is induced in the electrostatic image bearing member and as a result, the transfer medium is electrostatically attracted into intimate contact with the electrostatic image bearing member. The electric field then produced by the corresponding charge induced by the charge on the transfer medium is sufficiently stronger than the electric field produced by the corresponding charge of the electrostatic image on the electrostatic image bearing member, so that the particles of the developer move to the transfer medium to thereby complete the image transfer. Then, the transfer medium is forcibly separated from the electrostatic attraction to the electrostatic image bearing member by a separating means such as separating belt and separating pawl. Once separated, the transfer medium has charged, unfixed developer particles on one surface thereof and has, on the other surface, a charge of the opposite polarity to the developer imparted during the image transfer step. Therefore, if the transfer medium being conveyed is abruptly bent, the bent portion thereof will experience a sharp variation in electric field which will result in disturbance of the visible image due to the unfixed developer on the transfer medium. Usually, such disturbance of the visible image appears in the form of a circular series of spots in which the developing density is reduced. Such disturbance resulting in a reduced developing density renders illegible the reproduced image such as characters or the like, and would especially result in a quite undesirable copy image if the image to be reproduced is a tone image such as a photograph or the like.
The above-noted phenomenon becomes pronounced when the specific resistivity of the transfer medium itself is higher. For example, where the transfer medium used is plain paper having its specific resistivity increased to 10.sup.12 .OMEGA.-cm or more under the influence of the environmental humidity and other factors, or where the transfer medium used is formed of insulative resin such as polyester film or the like, the disturbance of the visible image is conspicuous.
This would be attributable to the following reason. Where the transfer medium is of a low specific resistivity, the charge retained by the developer particles deposited on the surface thereof is liable to be neutralized by the charge imparted by the voltage application during the image transfer, but if the resistivity becomes higher, there will occur no sufficient movement of the charge to neutralize the charge of the developer particles. Thus, for a transfer medium of high specific resistivity, conveyance will take place with charge retained by the developer particles on the surface thereof. If such a transfer medium is deformed by a bend of the conveyance route, the electric field formed by the surface charge of the transfer medium will be sharply varied so that the developer particles will be scattered away under the influence of that variation in electric field.
These problems may be overcome by the present invention.